Transmission network



Aug. 7, 1934. w. P. MASON TRANSMISSION NETWORK Filed March 17, 1933FILTER SECTION F /1. TE R SECTION FILTER SECTION F/L TE R SECTIONFREQUENCY /N VENTUR W P. MAS 0N FREQUENCY FREQUENCY A TTORNEY PatentedAug. 7, 1934 1,969,571 Ta NsMIssroN NETWORK VJarren Mason, West Orange,N. .L, assignorito Bell Telephone Laboratories, Incorporated, New York,N. Y, a corporation of New York p Application March 17, 1933, semi No.661,268 7 f 16 Claims. (01. 172F443 This invention relates to wavetransmission networks and more particularly to wave filters employing incombination piezoelectric crystals and inductance coils as impedanceelements.

An object of the invention is to improve the transmission and impedancecharacteristics of wave filters.

Another object is to reduce the attenuation distortion in thetransmission band of. wave 10 filters due to the inherent resistanceassociated with the component inductance coils.

Another object is to decrease the impedance irregularities at the inputterminals of a multisection Wave filter caused by reflection effects atthe junction points of the filtersections.

A further object is the elimination of longitudi-' nal currents from theoutput of a balanced wave filter.

A feature of the invention is a multi-section "2.0 wave filter in whichan ohmic resistance has been inserted between adjacent sections, and theload impedances between which the filter is connected have been modifiedin order to reduce'the reflection eifects otherwise caused by theeffective resistance inherent in the reactance ele ments comprising thefilter.

The wellknown property of resonance in piezoelectric crystals and theirextremely small energy dissipation make them highly suitable for use asimpedance elements in wave transmission networks. When they are thusused in wave filters it has been found necessary to associate inductancecoils with the crystals in order to broaden the transmission bandobtainable.- The introduction of'the more highly dissipative inductances tends to increase the transmission loss in; the transmissionrange and also to distort the attenuation characteristic in this range.

The attenuation distortion is the more objectionable of these twoeifects, since the increased transmission loss may easily be made up bythe use 'of a properly designed thermionic tube amplifier. Therefore. ifthe dissipation in the coils,

needed to widen the band of the filter, has onlybe considered aseffective at the ends of thesection, either in seriesor in parallel withthe filter.;

In accordance with the present invention these resistances areincorporated with the load im-;

coil resistances are effectively] 'inlseries or in shuntwith the filter.Also,.between filter sec-' tions these effective resistances areincorporated with added ohmic resistances in such a way as to form aconstant resistance attenuator of essentially the same impedance as thefilter. If the resistances are effectively in series with the filter rssMae 1 ems-E then a shunt resistanceis added between the sections; if theeiiective resistances are in shunt with the filten'the' added-resistance'is placed in series between the sections. In this'way the'filter isterminated in itscharacteristic impedance both at the ends and at thejunctions of the sectionsj As a result a constant loss independent offre'quencyjis added to'the attenuation characteristic of the filterwhich would be" obtainable if the elementswere dislsipatioriless; Also,the terminal impedance of the filter in the-transmission bandis mademuch more uniform, since the reflection efiects are eliminatedat theends of the filter and at the points where the sections are joinedp 7 Inthe case where the added resistance is connected in shunt, thisresistance maybe tapped and grounded atits center point, thus providinga low impedance'path to groundforundesired longitudinal currents whichmay be fiowing in the same direction along the two: sides 'of a balancedfilter. g Q I 1 The invention will'be more fully understood from thefollowing detaileddescription and by reference "to the accompanyingdrawing, *of which: I

Figs. land 2 are 'schematicsof typical'wave filter sections to which theinvention is ap; plicable; i- Figs. 3 and 4 show diagrammatically'twoem-- bodiments'o'f the'i nvention'; and i Figs. 5, 6 and '7 are diagramsto which reference is made in explaining the advantages of theinvention. 1 V

A typical wave filter section to which the invention is applicable isshown in Fig.1. 'The section consists of a lattice of piezoelectriccrystals, shunted at each end by condens'ersand with four'equalinductance coils L1 irrseries with the combination. Condensers are'alsoshunted 55. smaller or larger, depending upon whether the acrosstheftwo crystalsin series withthe' line. For a more detailed descriptionofthis'ty'pe; of filtersectionjreference is made to my copendingapplication, Serial No. 489,268, filed October 17, 1.936. The energydissipation in the crystals and the condensers will "ordinarily be verysmall in amount-and; therefore may safely be neglected The resistanceassociated with eachinductance L1 is effectively in series with eachcoil, as represented by the resistances R1 of Fig. 3. The value of theresistance R1 may be found by direct measurement or by well knownmethods of computation. In accordance with the invention each loadimpedance R5 is reduced in value to allow for the two series coilresistances R1. In equation form,

R5=Ro2R1 (1) where R0 is the characteristic impedance of the filter inthe transmission band. In this way, the

' filter is terminated at each end in its characteristic impedance, andtherefore, reflection effects at these points are eliminated.

Between filter sections "there will be two coil resistances in series ineach side of the line. If left uncorrected, there will thus be animpedance mismatch between the sections. In accordance with theinvention, however, an ohmic resistance Ra'is shunted across the linebetween the two junction points of the series coil resistances R1. Thevalue of R3 is so chosen that, in combination with the four resistancesR1, it forms a constant resistance attenuator of impedance R0. Inequation form When the resistance R3, evaluated in accordance withEquation (2), is added each filter section faces its characteristicimpedance at the junction of the sections, and reflection effects atthis point are thereby eliminated. The multi-section filter shownin Fig.3 will, therefore, have the attenuation characteristic which would beobtained if elements having no dissipation were used, with the additionof a constant loss over the entire extent of the frequency range. Thisadditional loss may readily be compensated, when necessary, by the useof a distortionless amplifier, as mentioned above.

As shown in Fig. 3 the added shunt resistance R3 may be tapped at itselectrical mid-point and connected to the ground G in order to eliminateundesired longitudinal currents which may be flowing in phase along thetwo sides of a balanced filter. Grounding the center point of thisresistance has no effect upon the loop current flowing in the filter,since the voltage producing this current is balanced with respect toground. However, a low impedance path to ground is thereby provided forthe longitudinal currents which fiow along both sides of the filter inthe same direction and utilize ground as the return path. Theseundesired longitudinal currents are in this way effectively eliminatedand do not appear in the output of the filter.

Another type of filter section to which the invention is applicable isthe one shown in Fig. 2,

which differs from the section shown in Fig. 1

only in that the inductance coils L2 are shunted across the ends of thefilter instead of being in series therewith. At any one frequency theresistance associated with the inductances may be represented aseiiectively in shunt across the input and output filter terminals, asshown by the resistances R2 of Fig. 4. Inorder to match the impedance oftheload to that of the filter, the load impedance R6 is increased invalue to compensate for the-shunt resistance R2. The required value forR6 is givenby the equation I Q 2 o R0 R2 R0 (3) where Rois thecharacteristic impedance of the filter section.

Where two of the sections join, a pair of equal resistances R4 areinserted in series between the sections. The value of the resistance R4is given by the equation o z R4 R22 R02 When these modifications havebeen made the filter sections shown in Fig. 4 are terminated at each endin their characteristic impedance and, as a result, reflection effectsat the ends of the filter and between sections are eliminated. Thefilter will then have the attenuation characteristic of a filtercomposed of dissipationless elements, with a small added loss which isconstant with frequency. At the same time, the terminal impedance of thefilter will be more uniform in the transmission band.

Some of the advantages of the application of the invention to a bandpass filter composed of two sections of the type shown in Fig. '1 areillustrated by the diagrams of Figs. 5, 6 and '7. Fig. 5 shows theattenuation-frequency characteristic of the filter in the transmissionband. The dotted line curve,gives the attenuation of the filter, whenshunt resistance R3, determined according to Equation (2), is added. Itis apparent from a comparison of the two. curves that the attenuationdistortion in the transmission band of the filter has been eliminated.The small amount of added-loss may easily be made up in an associatedamplifier. The dotted line curve and the solid line curve of Fig. 6 givea comparison of the terminal resistance of the two-section filter ofFig. 3, respectively, before and after the application of the invention.The two curvesof Fig. 7 show the improvement in the terminal reactanceof the filter under similar conditions. It will be noted that theimpedance of the filter in the transmission band is made considerablymore uniform by the-application of the invention. As a result, thereflection efiects which occur between the filter and the load aregreatly reduced.

The inductance coils L1 and L2 of Figs. 1 and 2 are shown as locatedexternal to the latticeportion of the filter sections but these coilsmay, of course, be incorporated within the lattices and the principlesof the invention may be applied equally the values of the variousresistances in accord-.

ance with Equations (1), (2), (3) and (4).

What is claimed is: r

1. In combination, a multi-sectioh wave filter and an attenuatorlocated'between two sections of said filter, the characteristicimpedance'of said attenuator being substantially equal to thecharacteristic impedance of said filter, and at least one of thecomponent resistances of said attenuator being formed by the effectiveresistance inherent in the elements comprising said filter.

2. In combination, a wave filter having a plurality of-sections and aconstant resistance attenuator located between two adjacent sections ofsaid filter, the characteristic impedance of said attenuator beingsubstantially equal to the characteristic impedance of the sections ofsaid filter, and at least one of the component resistances whichcomprise said attenuator being formed by the effective resistanceinherent in the elements which comprise the sections of said filter.

3. In combination with a multi-section wave filter, a resistanceinserted between two adjacent sections of said filter, the magnitude ofsaid re sistance depending upon the characteristic impedance of saidfilter and also upon the effective resistance inherent in the reactanceelements which comprise the sections of said filter, whereby each ofsaid two adjacent sections at their junction point faces an impedancewhich is substantially equal to the characteristic impedance of thesection.

4. In combination, two wave filter sections con- '1 nected in tandem anda resistance shunted across the line at the junction of said twosections, the magnitude of said resistance depending upon thecharacteristic impedance of said filter sections and also upon theeffective resistance in- 'I herent in the reactance elements which formsaid sections, whereby reflection effects at the junction of said twosections are reduced.

5. In combination, two wave filter sections connected in cascade and aresistance connected in II series between said two sections, themagnitude of said resistance depending upon the characteristic impedanceof said filter sections and also upon the inherent resistance of thereactance elements comprising said sections, whereby the at- ..tenuationdistortion in the transmission band of said filter is reduced.

6. In combination, a wave filter and a load impedance connected to theterminals of said filter, the magnitude of said load impedance differingTI from the characteristic impedance of said filter by an amount whichdepends upon the inherent effective resistance of the reactance elementscomprising said filter, whereby said filter is terminated in itscharacteristic impedance, thereby reducing the attenuation distortionand impedance irregularities in the transmission band of said filter.

7. In combination, a wave filter and a load impedance associated withsaid filter, the magnitude "of said load impedance being smaller thanthe characteristic impedance of said filter by an amount equal to theinherent resistance of the reactance elements comprising said filter andeffectively in series with said filter, whereby said filter isterminated in its characteristic impedance.

8. In combination, a wave filter and a load impedance connected to theterminals of said filter, the magnitude of said load impedance beinglarger than the characteristic impedance of said filter by an amountwhich depends upon the inherent resistance of the reactance elementscomprising said filter and effectively in shunt with the terminals ofsaid filter, whereby said filter is terminated in its characteristicimpedance.

9. In combination, a multi-section wave filter comprising piezoelectriccrystals and inductance coils as reactance elements, and an attenuatorlocated between two sections of said filter,

at least one of the component resistances of said attenuator beingformed by the-effective resistance inherent in the inductance coilscomprising said filter. A a

10. In combination, a multi-section wave filter comprising piezoelectriccrystals and inductance coils as reactance elements, and a resistance-'inserted' between two adjacent sections of saidfilter, the magnitudeof said resistance depend ing upon the characteristic impedance of saidfilter and the effective resistance inherent in said inductance coils,whereby each of said two adjacent sections at their junction point facesan impedance which is substantially equal to the characteristicimpedance of said filter. I

11. In combination, two wave filter sections connected in seriesrelation, and a resistance connected in shunt with the line at thejunction of said two sections, said filter sections comprisingpiezoelectric crystals and inductance coils as reactance elements, andthe magnitude of said resistance depending upon the characteristicimpedance of said filter sections and also upon the magnitude of theeffective resistance inherent in said inductance coils, whereby theattenuation distortion otherwise present in the transmission band ofsaid filter is reduced.

12. In combination, two wave filter sections connected in tandem, and aresistance connected in series between said two sections, said filtersections comprising piezoelectric crystals and inductance coils asreactance elements, and the magnitude of said resistance depending uponthe characteristic impedance of said filter sections and also upon themagnitude of the effective resistance inherent in said inductance coils,whereby refiection loss is eliminated in the transmission band of saidfilter.

13. A frequency selective wave transmission system comprising a wavefilter having two pairs of terminals, and transmission devicesrepresenting terminal impedances'connected to said pairs of terminals,said filter comprising piezoelectric crystals and inductance coils asreactance elements, and the magnitude of at least one of said terminalimpedances differing from the charac teristic impedance of said filterby an amount which depends upon the inherent effective resistance ofsaid inductance coils, whereby said filter is effectively terminated inits characteristic impedance.

14. In a frequency selective transmission system, a multi-section broadband wave filter having piezoelectric crystals and inductance coils asreactance elements, a resistance inserted between two adjacent sectionsof said filter, and transmission devices representing terminalimpedances connected to the ends of said filter, the magnitude of atleast one of said terminal impedances differing from the characteristicimpedance of said filter by an amount which depends upon the inherentefiective'resistance of said inductance coils, and the magnitude of saidinserted resistance depending upon the characteristic imped-- 15. Incombination, two lattice type wave filter sections connected in cascade,and a resistance inserted between said two sections, said filtersections comprising piezoelectric crystals and in-- ductance coils asreactance elements, and the magnitude of said inserted resistancedepending upon the characteristic impedance of said filter sections andalso upon the magnitude of the effective resistance inherent in saidinductance coils, whereby each of said two sections at their junctionfaces an impedance which is substantially equal to the characteristicimpedance of said filter, thereby reducing the attenuation dis tortionand impedance irregularities otherwise present in the transmission bandof said filter due to theinherent effective resistance of saidinductance coils.

16. In combination, two balanced wave filter sections connected intandem and a resistance shunted across the line at the junction of saidtwo sections, said resistance being grounded at its electricalmid-point, and the magnitude of said resistance being determined withrespect to the characteristic impedance of said filter sections and alsowith respect to the effective resistance inherent in the reactanceelements which comprise said sections, whereby the attenuation andimpedance characteristics of the filter are improved and longitudinalcurrents are effectively drained off and thereby eliminated from theoutput of the filter.

WARREN P. MASON.

